In film stretching machines in the prior art it is known to arrange chain links, which are connected together with one another to form a chain, in an endlessly circulateable manner in the film stretching machine. It is typical to use two parallel-running systems of chains that consist of chain links on which clamps for the film edge, so-called tenter clips, are arranged. The film is transported through the film stretching machine with the aid of the tenter clips. In that regard, the two chain systems are arranged so that their spacing distance transversely to the film transport direction gradually increases. Drive and deflection devices for each of the two chain systems are provided at the inlet and at the outlet of the machine. After the release of the film by the tenter clips at the outlet of the machine, the chain links with the tenter clips return back to the inlet of the machine outside of the film transport path.
The chain links run at a high speed and at an increased temperature. Therefore the connection joints between the individual chain links are subject to an undesired wear. Due to this wear, the spacing distance between the chain links becomes larger. A stretching or elongation of the respective chain arises, and upon exceeding certain magnitudes leads to operating disturbances and to faults in the film stretching process. In order to avoid these disturbances, it is necessary to carry out maintenance measures at the proper time. In order to determine this time point for the maintenance measures, measuring systems are known in the prior art, with which a stretch or elongation of the chain can be determined during the running operation of the chain.
Such a measuring apparatus is shown, for example, by the JP 2010 190578 A1. This measuring apparatus works with two sensors that probe and sense the contours of the chain links or the tenter clips. A geometric length for a certain number of chain links is determined from the time difference between two sensor signals and the current chain speed. The apparatus for monitoring a conveyor belt according to DE 199 02 759 A1 also operates with the evaluation of time differences between sensor signals. The JP 07-268843 A also discloses an apparatus for determining a stretch or elongation of a chain. Therein, however, an intensity difference of signals is evaluated instead of a detection time difference of two sensors. The intensity difference arises due to different positions of elements for signal generation that are applied or mounted on the chain. The EP 1 873 088 A1 describes an apparatus for determining wear-induced changes on the running surface of a belt. In that regard, magnetic signal generators that are embedded in the belt cause certain magnetic force progressions or patterns, which are detected and electronically evaluated by one or more sensors.